The present application relates to a light emitting composition having a phosphor, a light source device having such a light emitting composition, and a display device having such a light source device.
A phosphor has a feature such that the wavelength band of light emitted from the phosphor changes according to the kind or composition of the components of the phosphor. Accordingly, the phosphor has received attention as a light emitting material in which a desired wavelength band of emitted light can be selected from a wide wavelength range.
On the other hand, it is expected to apply the phosphor having such a feature to a thin display device called an FPD (flat panel display) such as a liquid crystal display, plasma display, and organic EL (electroluminescence) display.
Since a light emitting configuration for obtaining white light by the combination of a YAG:Ce phosphor to be excited by blue light and an LED (Light Emitting Diode) as a blue light source was proposed (see Japanese Patent No. 2927279, for example), the above attention and expectation have been increasing on a phosphor to be excited by blue light.
Such a light emitting configuration obtained by the combination of a phosphor to be excited by blue light and a blue light source has been considered to be suitable for a light source device adapted to be used in a display device (e.g., a backlight used in a liquid crystal display) (see JP-A-2004-505172, for example).
According to this light emitting configuration, a drive circuit can be simplified as compared with a configuration such that all of the blue, green, and red colors are obtained by direct drive type light sources such as LEDs. Further, according to this light emitting configuration, a deterioration of peripheral members (resin members, LED chips, etc.) can be suppressed as compared with a configuration using near-ultraviolet light as excitation light. Owing to these advantages, the combination of a blue light source and a phosphor is considered to be suitable for a light source device in a display device.
However, most of phosphors to be excited by blue light have a disadvantage such that the moisture resistance is low and the characteristics are therefore deteriorated by the moisture contained in the air (atmosphere).
In general, a light source device is difficult to clear a reliability test under the conditions of ordinary temperature and ordinary humidity and is difficult to additionally clear a reliability test under more stringent conditions (e.g., a temperature of 40° C. or more and a humidity of 90% RH (relative humidity) or more). However, since the phosphor to be excited by blue light has low moisture resistance, the above severe reliability test is difficult to be cleared. For example, in the case that an alkaline earth metal sulfide (CaS:Eu) as the phosphor described in JP-A-2004-505172 is mixed with epoxy resin and a blue LED is potted with this mixture to perform a reliability test under the conditions of 45° C. and 95% RH, the emitted light intensity decreases by about 60% after 173 hours as shown in FIG. 5. This result is insufficient as long-term reliability of products.
To solve this problem on the moisture resistance of the phosphor, it has been proposed to deposit glass on the surface of the phosphor, thereby improving the moisture resistance (see Japanese Patent Laid-open No. 2006-52354, for example).
However, the present inventors have found that the deposition of glass is difficult to improve the moisture resistance of the phosphor. More specifically, the present inventors have found that the influence of the deposition of glass on the phosphor changes according to the condition and degree of the deposition and that the moisture resistance is possibly reduced when the amount of deposited glass is small (e.g., glass is discretely deposited on the surface of the phosphor).
The present inventors have performed an environmental test under the conditions of 25° C. and 75% RH by using a phosphor on which glass is discretely deposited.
As the test result, although the test conditions are less stringent than those of the reliability test mentioned above with reference to FIG. 5, it was determined that the emitted light intensity of the phosphor is reduced with time irrespective of whether or not the deposited glass is present as shown in FIG. 6. Further, as apparent from FIG. 6, the rate of deterioration of the phosphor is higher in a sample having the deposited glass (broken line y) than in a sample having no deposited glass (solid line x).